/**
 * Implement infinite scrolling
 * - Inspired by: http://ravikiranj.net/drupal/201106/code/javascript/how-implement-infinite-scrolling-using-native-javascript-and-yui3
 */
 
(function() {
    var isIE = /msie/gi.test(navigator.userAgent); // http://pipwerks.com/2011/05/18/sniffing-internet-explorer-via-javascript/
    
    this.infiniteScroll = function(options) {
      var defaults = {
        callback: function() {},
        distance: 50
      }
      // Populate defaults
      for (var key in defaults) {
        if(typeof options[key] == 'undefined') options[key] = defaults[key];
      }  
      
      var scroller = {
        options: options,
        updateInitiated: false
      }
      
      window.onscroll = function(event) {
        handleScroll(scroller, event);
      }
      // For touch devices, try to detect scrolling by touching
      document.ontouchmove = function(event) {
        handleScroll(scroller, event);
      }
    }
    
    function getScrollPos() {
      // Handle scroll position in case of IE differently
      if (isIE) {
        return document.documentElement.scrollTop;
      } else {
        return window.pageYOffset;
      }
    }
    
    var prevScrollPos = getScrollPos();
    
    // Respond to scroll events
    function handleScroll(scroller, event) {
      if (scroller.updateInitiated) {
        return;
      }   
      var scrollPos = getScrollPos();
      if (scrollPos == prevScrollPos) {
        return; // nothing to do
      }
      
      // Find the pageHeight and clientHeight(the no. of pixels to scroll to make the scrollbar reach max pos)
      var pageHeight = document.documentElement.scrollHeight;
      var clientHeight = document.documentElement.clientHeight;
      
      // Check if scroll bar position is just 50px above the max, if yes, initiate an update
      if (pageHeight - (scrollPos + clientHeight) < scroller.options.distance) {
        scroller.updateInitiated = true;
    
        scroller.options.callback(function() {
          scroller.updateInitiated = false;
        });
      }
      
      prevScrollPos = scrollPos;  
    }
  }());